Thread: sbc running rough

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I have a 68 350 i'm running in my truck.I had tbi on it and run little rough becuz of the aftermarket cam i had in it.So i put a carb on it and now it runs rough all the time.
it has a 600 cfm 4bbl carb. hei dis. It idle ok little rough but as soon as i open it up it starts to fall on its face.it sounds likes it only running on 7 cly when i give it gas.Put it in gear and die's.It has spark knock if i give it gas whlie it is in gear.
I adjust all the lifters when i put the new intake on with the carb.

I try a diff dis and run the same.When i check the timing with the light the pointer is off by about 15* it is not were itshould be and i think it is from the blanace being newer. My vac is about 15 low but needle is satying still.

I hope some one can help.I have never had so much trouble before

you say your "pointer is off by about 15" degrees.....are you saying that the timing tab is not positioned properly or that your timing is out of whack?

In my unqualified opinion, it sounds like your timing is way out of whack in conjunction with other problems. Whatever existed before was exacerbated by the installation of the carb. Not necessarily by the carb itself, but through the installation process.

You'll need to get some firm baselines and provide more info. What are the specs on your aftermarket cam?

btw: welcome to CHR....there are a lot of really smart people here (I am not included) and in my opinion, the better you are at defining your problem and clearly stating relevant details in anticipation of their questions, the more likely you are to get the help you need.

There are a few things I would question..
1) Is the distributor in the right place ( not 1 or more teeth off)?

2) What are the chaces you have a vacume leak in the intake or under the
carb?

3) All of the above?

4) How many miles on engine may be timing chain??


OH I didn't notice till I posted Welcome to CHR If it can be fixed there are people here that can tell you how.

What i mean about the pointer is.
it should be at the 1:30 almost 2 o'clock spot with the timing mark and what it is when the timming light is on it is at the 10:30-11 o'clock
But when i turn the engine over tdc is right with the mark.

I did check for leaks.did not find any.I cover up the carb with my hands and check it with brake clean nothing.
Being off a tooth i do not think. cuz it runs pretty good.at idle.
I put a new chain on the enigne before i put it in this truck and maybe 30-40 miles tops.Miles on the engine is unkown maybe 50000 tops

The truck had some power just not what it should of have when the TBI was on it.

I did check the compression in the front four 1,2,3,4 cly i get about 135psi
i can not check the back four becuz of the header are in my way

Welcome aboard....

Sounds mostly like too much timing advance (spark knock under load, etc). Although, things can get more complicated with EFI computers and what-not that I can't really speak to. (computer controlling timing?)

Assuming good ol' carb'd motor you should have about 8-12* (BTDC)timing at idle without vac. advance and somewhere in the ballpark 32-38* (BTDC) advance at 2500-3000rpm. If you're shooting "11 o'clock" against a "2 o'clock" timing mark at idle that's close to 45* and way too much. Disconnect and plug any vacuum advance lines and shoot it again... you'll want to loosen the dist hold down and rotate the body clockwise to reduce your advance....

If there are computers involved, I can't help but I'm sure there are others around that will chime in....

Good luck,

Chris

No computer.And the timming mark is there without the vacuum hooked up.
If i turn the dis either way it will not run good.if at all.

Hmmmmm... how'd you set your idle mixture? Maybe air/fuel off enough that it needs more advance at idle (too rich??). I'm grasping here.... hopefully others can pop in with a suggestion or two. Have you checked your plugs/wires? Still sounds like too much time....

I have adjust the idle mix a few times no change.The wires are new the plugs were new and i just clean them.the gap is .40...It does seem rich and i change the jets down to .60 the smallest ones i have in my set.
I just had a buddy down off a diff carb to test with just for the heck of it.But it is a 390cfm 4bbl.
I will try anything before i start pulling valve covers and intakes and timming covers to check everything else.

I know you said it is in right butif it were me I'd remove the dr side valve cover get # one TDC and see where you are with your timing mark. You have to start some where. Thats where I'd start

yeah that was the next step i will do tonite.

"But when i turn the engine over tdc is right with the mark."

How did you determine this? What method did you use?

Well thats what I'm trying to find out. You have to start somplace and this is as good or better than anyother.

i pulled the #1 plug and stuck a Screwdriver in the hole while i turn it over